Temperature insensitive Mach-Zehnder interferometers and devices

ABSTRACT

A Mach-Zehnder interferometer having two optical couplers interconnected by two optical fibers at least one of which is temperature insensitive. In use, temperature induced changes in the geometrical length and refractive index of the temperature insensitive fibers offset each other so that the optical path length of the fiber is unaffected by the temperature change. Where two temperature insensitive fibers are included these may be of the same or of different lengths. The interferometer may be used in a Dense Wavelength Division Multiplex system.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of application Ser. No. 09/920,050filed Aug. 2, 2001, now U.S. Pat. No. 6,778,278.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical fiber devices andparticularly to fiber based Mach-Zehnder interferometers (MZI), whichare made insensitive to temperature changes and devices employing thesame.

2. Prior Art of the Invention

Optical filters are frequently used in modern optical communicationsystems such as Dense Wavelength Division Multiplex (DWDM) systems. Inthese systems, a number of data channels share a single optical fiber astheir transmission media and use a unique wavelength of light as theirchannel signature.

An optical waveguide MZI is composed of two optical splitters/couplers.Two lengths of optical waveguides or arms connect them to each other.When the arms of the MZI have different lengths, we have a so-calledasymmetric Mach-Zehnder interferometer(AMZI). The optical waveguidesreferred to here are optical fibers with circular cross sections and/orplanar optical waveguides with non-circular cross sections. A MZI madeof optical fibers is called an all-fiber MZI.

Asymmetric MZIs show a periodic response as a function of wavelength.The period is a function of the length difference between the arms ofthe interferometer. As the length difference increases, the channelspacing decreases and, therefore, the wavelength selectivity increases.Asymmetric MZIs, once connected to each other as inter-leavers, canmultiplex or de-multiplex a large number of optical signals of differentwavelengths such as the standard ITU (International TelecommunicationsUnion) grid wavelengths. An optical inter-leaver can separate the oddand even channels from a WDM signal consisting of several wavelengths.

One problem associated with fiber based AMZIs is their sensitivity totemperature, and the greater the length difference, the more severe isthe problem. The problem originates mainly from a temperature-inducedchange in the optical path length of the fiber. As the temperaturechanges, the refractive index and the geometrical length of the fiberchange. Consequently, a difference in the optical path lengths of twoarms is created.

Several temperature compensation methods have been proposed to solve thetemperature sensitivity of these photonic devices. However, most ofthese methods work within a limited range of temperatures and cannot beapplied easily to the asymmetric MZI cases where large differences inthe optical paths exist. Active temperature compensation of photonicdevices is typically carried out by maintaining the temperature of thefibers' environment above a chosen temperature (e.g., above 60° C.).This is achieved by including a heater controller inside the package ofthe device. However, the high power demands of the active temperaturecompensation and its low reliability have made the search for passivemethods an on-going effort within the photonic industry.

Temperature insensitive fibers can be built by a method that, forexample, has been disclosed in the U.S. Pat. No. 5,018,827. In the namedpatent, an insensitive optical fiber is produced when an optical fibercore made of a first material is enclosed within a cladding made of asecond material having a different coefficient of thermal expansion.

In a recent U.S. Pat. No. 6,081,641, a passive temperature compensatingmethod is presented for a fused-fiber DWDM system. In this invention,two dissimilar materials with different thermal expansion coefficientsare used to construct a fixture containing the DWDM device. By usingthis structure, it is possible to artificially create a negativecoefficient of thermal expansion. The DWDM device is typically assembledon a pre-stressed fixture. However, the device can also be built undertension and then assembled on the relaxed bi-substrate fixture. In theformer design, the whole assembly can exert tension on, or releasetension from, the fiber. Temperature compensation is then established byadjusting the applied tension on the fused-fiber DWDM. It is shown that,as tension is relieved, the thermal drift due to an increase intemperature is compensated. Conversely, by increasing tension,wavelength shifts due to a decrease in temperature are compensated. Byusing such a temperature-compensating device, a bulky package isinevitable. In addition, dimensional design and choice of material canbe demanding requirements.

An object of this invention is to provide a novel temperatureinsensitive asymmetric fiber based MZI.

SUMMARY OF THE INVENTION

Optical filters with sharp wavelength characteristics are vitalcomponents of WDM technology. Interferometer devices, and in particularfiber based interferometer devices such as the MZI, show usefulfiltering characteristics, are easily expandable, and exhibit lowinsertion loss. A fiber based optical MZI consists of two opticalcouplers or splitters with predetermined coupling or splitting ratiosconnected together through two lengths of optical fiber. In order todecrease the channel spacing between two adjacent channels of aninterleaver response, the length difference (Δl) between the two armsshould increase. As a result, the optical path length difference alsoincreases, generating a higher sensitivity within the MZI tofluctuations in its temperature.

The challenge lies in correctly achieving the desired channel spacing.This is accomplished by measuring the correct Δl between the two armsconnecting the two couplers of the MZI. As a result of the differentoptical paths between the two arms of the two couplers, a sinusoidalwavelength response can be obtained with low polarization dependence andlow insertion loss. Using a precision reflectrometer or an opticalspectrum analyzer, the difference between the two arms of the MZI can bemeasured to within ±10 μm.

The object of this invention is consistent of two parts. In one part, atemperature insensitive MZI may be made using a specialty fiber asdisclosed in U.S. Pat. No. 5,018,827. An insensitive optical fiber canbe tailored such that temperature-induced changes in its geometricallength and in its refractive index offset each other in such a fashionthat the optical path length is, for all intents and purposes,independent of temperature. By carefully choosing two different glassesfor the core and cladding, and by appropriately adjusting their radii,the observed center wavelength shift sensitivity of the MZI filter dueto temperature variations can be eliminated. It is noted this specialtyfiber that may contain different dopant concentrations from that in theregular single mode fiber can be fusion spliced to a regular fiber withminimum loss. In the second part of this invention, a layer of aproperly selected material is deposited onto a small section of one armor both arms of the MZI to compensate for the temperature-inducedvariations.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred exemplary embodiments of the present invention will now bedescribed in detail in conjunction with the annexed drawing, in which:

FIG. 1 shows general structure of a prior art asymmetric Mach-ZehnderInterferometer (MZI);

FIG. 2 illustrates the transfer function of the asymmetric MZI periodicfilter of FIG. 1;

FIG. 3 shows a typical single optical fiber cross-section as used in theMZI of FIG. 1;

FIGS. 4 and 5 show in cross-section and perspective the temperaturecompensating deposited coating layer onto the fiber according to thepresent invention.

FIG. 6 show the general structure of an asymmetric MZI incorporating thetemperature compensated fiber of the FIGS. 4 and 5;

FIG. 7 shows the AMZI of FIG. 6 but with the temperature compensatedfiber in only one arm; and

FIG. 8 displays the length of the coated section vs. coating thicknessfor the preferred examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, it depicts a prior art AMZI with couplers 1 and 2interconnected by two optical fibers F1 and F2, F2 being slightly longerthan F1 to provide an additional differential delay ΔL, thus providingan asymmetric MZI (and periodic filter) without temperaturecompensation. The AMZI is, therefore, sensitive in its filteringtransfer function, shown generally in FIG. 2, to variations in ambienttemperature. The fibers F1 and F2 have a cross-section as shown in FIG.3 with a core of radius R_(core) and a cladding of radius R_(cladding),the Coefficient of Thermal Expansion (CTE) being x_(core) andx_(cladding), respectively.

In FIG. 4 is shown a cross-section of a temperature compensated fiberhaving a long part of its length a surrounding coating of, generally,radius R₁ and R₂ and a CTE x_(coat). Such fiber section is shown inperspective FIG. 5 and, generally, two such section l₁ and l₂ as shownin FIG. 6, are inserted as portions of the uncompensated fibers F₁ andF₂, thus providing a temperature compensated (insensitive) AMZI, havinga stable transfer function as shown in FIG. 2.

An alternative embodiment of FIG. 6 is shown in FIG. 7, where only thefiber arm F₂ has inserted therein a temperature compensated section l₂,which also of sufficient length to provide the requisite delay ΔL.

FIGS. 6 and 7 thus depicts the novel temperature insensitiveMach-Zehnder Interferometer (MZI). This structure could be used in manyother optical systems that use an MZI. This is particularly significantin Dense Wavelength Division Multiplexing (DWDM) subsystems such asoptical interleavers, multiplexer/demultiplexers and filters which arebased on MZIs.

An AMZI consists of two optical 3-dB (50:50) couplers interconnectedwith two fiber arms having different optical path lengths, one arm beinglonger than the other by ΔL. Once the temperature changes, the lengthsof the optical paths of the two arms also change. Since the two arms donot have the same length, one arm experiences more changes than theother. It should be noted, however, that the temperature dependency isnot only due to the geometric path length expansion or contraction butalso due to the change in the refraction index of the fiber. The aim isto compensate for both effects and consequently the changes caused bytemperature variations.

Referring to FIG. 3, the fiber core radius is indicated by R_(core) thecladding radius by R_(cladding). The Coefficient of Thermal Expansion(CTE) of the core material is α_(core) and that of the cladding materialis α_(cladding). The effective CTE of the optical fiber isα_(fiber)=(α_(core) A _(core)+α_(cladding) A _(cladding))/(A _(core) +A_(cladding)),  Eq. 1where A_(core) and A_(cladding) are the cross-sectional areas of thecore and cladding, respectively. The above formula is simply a weightedaverage of the two coefficients of thermal expansion. Replacing A_(core)by πR_(core) ² and A_(cladding) by π(R_(cladding) ²−R_(core) ²) weobtainα_(fiber)=(R _(core) /R_(cladding))²(α_(core)−α_(cladding))+α_(cladding).  Eq. 2

The optical path L_(opt) of an optical fiber of geometric length L_(geo)and refractive index n isL_(opt)=nL_(geo).  Eq. 3

Consequently, the change of the refractive index or geometric length canaffect the optical path length as follows.ΔL _(Opt) =Δn·L _(geo) +n·ΔL _(geo)  Eq. 4

In this equation, Δn is the thermal change in the refractive index for atemperature change of ΔT degrees, which is equal to (dn/dT)ΔT.Similarly, ΔL_(geo) indicates the thermal expansion or contraction ofthe geometric length of the fiber for ΔT, i.e. ΔL_(geo)=(dL_(geo)/dT)ΔT.Replacing them in the above equation, we can get

 ΔL _(Opt) =[L _(geo)(dn/dT)+n(dL _(geo) /dT)]ΔT  Eq. 5

We also know that in the linear region of the thermal expansion of thegeometric length of the fiber dL_(geo)/dT=α_(fiber)L_(geo). Therefore,ΔL _(Opt)=[(dn/dT)+nα _(fiber) ]L _(geo) ΔT.  Eq. 6

From the above equation, if (dn/dT)+nα_(fiber)=0, or(dn/dT)=−nα_(fiber), then L_(opt)=0, i.e. optical path length does notchange with temperature. The typical values for α_(fiber) are in therange of 10⁻⁷ (° C.⁻¹), while typical values for dn/dT are usually inthe range of 10⁻⁶ (° C.⁻¹). Therefore, there is a chance to select someof the parameters of the fiber, such as the core or cladding radii, coreor cladding material, and so on, to provide a temperature insensitivefiber. The present invention, however, provides a simpler method tocompensate the temperature sensitivity of the asymmetric MZI.

According to the present invention, a layer or coating of a selectedmaterial is deposited onto a small portion along the length of theoptical fiber constituting one arm or both arms of the MZI. There are anumber of advantages to this method; some of them are discussed here.This method eliminates the complexity of specialty fiber manufacturingneeded for a temperature insensitive fiber. Secondly, the depositedmaterial can be selected from a wider range of materials by varying thethickness of the deposited layer. Such method is not as complicated asthe fabrication of a specialty temperature insensitive fiber. Finally,the method can be easily adapted to different fiber types.

As shown in FIGS. 4,5 and 6, the general case is where a layer of aproperly selected material is deposited as coating onto each arm of theMZI. For the shorter arm, the length of the coating region is shown byl₁, the radius of the resulting cross-sectional radius and area by R₁and A₁, respectively. Similarly, l₂, R₂ and A₂ show the length,resulting cross-sectional radius and area for the longer arm. The CTEfor the coating material on the shorter arm is α_(coat(1)) andα_(coat(2)) for the longer arm. The effective CTE for these regions canbe calculated byα_(i)=(α_(core) A _(core)+α_(cladding) A _(cladding)+α_(coat(1)) A ₁)/(A_(core) +A _(cladding) +A _(i)),  Eq. 7where i=1,2. In the above equation, α₁ and α₂ are the effective CTE forthe coated region of the shorter and longer arm, respectively. Againreplacing the cross-sectional areas we get toα_(i)=(R _(core) /R _(i))²(α_(core)−α_(cladding))+(R _(cladding) /R_(i))²(α_(cladding)−α_(coat(i)))+α_(coat(i)′),  Eq. 8and i=1,2.

Now assume the geometric length of the shorter arm of the MZI to beL_(g1), and the longer arm to be L_(g2)=L_(g1)+l₀. As discussed before,in order to compensate for the temperature changes the followingcondition must satisfy.ΔL_(1Opt)=ΔL_(2Opt)  Eq. 9

Replacing each side for a ΔT temperature change, we find(dn/dt+nα _(fiber))(L _(g1) −l ₁)ΔT+(dn/dT+nα ₁)l ₁ ΔT=(dn/dT+nα_(fiber))(L _(g1) +l ₀ −l ₂)ΔT+(dn/dt+nα ₂)l ₂  Eq. 10

If we rearrange and simplify the equation, we can write it as(α₁−α_(fiber))nl ₁=(α₂−α_(fiber))nl ₂+(dn/dT+nα _(fiber))l ₀  Eq. 11

If we assume the coating length is on one of the arms, the aboveequation gives the coating length on the other arm of the MZI. For thesimplest case, we deposit on only one arm. In that case, we set thelength of the coating region on one of the arms to zero.

If the coating section is on the shorter arm, then $\begin{matrix}{{l_{2} = 0},{l_{1} = \frac{( {\frac{\mathbb{d}n}{\mathbb{d}T} + {n\quad\alpha_{fiber}}} )l_{o}}{n( {\alpha_{1} - \alpha_{fiber}} )}}} & {{{Eq}.\quad 12}\text{-}1}\end{matrix}$

If the coating section is on the longer arm, then $\begin{matrix}{{l_{1} = 0},{l_{2} = \frac{{- ( {\frac{\mathbb{d}n}{\mathbb{d}T} + {n\quad\alpha_{fiber}}} )}l_{o}}{n( {\alpha_{2} - \alpha_{fiber}} )}}} & {{{Eq}.\quad 12}\text{-}2}\end{matrix}$

It should be noted that usually dn/dT is positive and greater inabsolute value than nα_(fiber). As a result the nominator value in Eq.12-2 is negative. In this case, Eq. 12-1 gives

-   -   α_(fiber)=α_(core)=5.6×10⁻⁷ (/° C.)    -   α_(coat(1))=2×10⁻⁶ (/° C.)    -   R_(core)=8 micrometer    -   R_(cladding)=125 micrometer    -   R₂=(125+50)=175 micrometer    -   α₂=1.27×10⁻⁶ (/° C.)    -   l₂=13.16 mm    -   l₂=0

If we increase the thickness of the coating layer to 0.1 mm (100micrometer), 0.5 mm (500 micrometer), and 1 mm (1000 micrometer), weobtain the following results.

Coating R₂ = (125 + 100) = 225 micrometer thickness α₁ = 1.56 × 1⁻⁶ (/°C.) 0.1 mm l₁ = 9.32 mm Coating R₂ = (125 + 500) = 625 micrometerthickness α₁ = 1.94 × 10⁻⁶ (/° C.) 0.5 mm l₁ = 6.72 mm Coating R₂ =(125 + 1000) = 1125 micrometer thickness α₁ = 1.98 × 10⁻⁶ (/° C.) 1 mml₁ = 6.53 mm

In FIG. 8, l₂ values for different coating thicknesses are plotted forthe above parameters for an Invar alloy (α=2×10⁻⁶ (° C.)⁻¹). We see thatfor thick layers of coating, the length of the coating region gets to alimit, which is around 4.9 mm for the above example.

Similar calculations can be carried out to find thickness and length ofthe coating section for the case of the deposition on the shorter arm ofthe MZI. It is apparent that a combination of depositions on both armscan also be done. In this case, the length of coating on one of the armsdepends on the other one. As a result, one of the lengths (i.e. l₁ orl₂) is the free parameter.

In one embodiment where an insensitive fiber is used instead of a coatedfiber, the two arms of the two couplers are cut into equal lengths andare fusion spliced to two insensitive optical fibers with apredetermined Δl. By cutting the two insensitive fibers to differentlengths, an optical path length difference is produced. Using a fibercleaving stage equipped with a micro-positional fixture it is possibleto make a precise Δl between the two arms of an MZI. Polishing the fiberto obtain the desired channel spacing before the arms are spliced toform the MZI achieves the final length adjustment

In another embodiment, the two arms of one coupler are cut as close inlength to each other as possible and are spliced to two arbitrarylengths of the insensitive fiber. The new coupler formed is then cut tothe desired Δl and fused to the two equal arms of the other coupler.

Another embodiment of a fiber based insensitive asymmetric MZI is madeof two couplers in which the insensitive fiber is used only in one ofthe arms of the MZI. The length of the insensitive fiber, in this caseprecisely equals the predetermined Δl. The other arm of MZI made ofconventional single mode silica fiber will then be fusion splicedtogether to form the MZI.

By forming an insensitive MZI, a complex bimetallic packaging structurefor passive temperature compensation is not needed, nor is an activemethod necessary. The use of expensive composite materials in thepackaging of the device is eliminated as well. An insensitive MZI ofthis invention can be easily made to any desired Δl. The novel design ofthis invention easily provides higher Δl and thus higher channel numberwithout the problem of temperature sensitivity due to different-opticalpath lengths.

1. A Mach-Zehnder interferometer comprising two optical couplersintercommunicated together by two optical fibers at least one of whichis a temperature insensitive fiber in which temperature induced changesin the geometrical length and refractive index of the temperatureinsensitive fiber offset each other whereby the optical path length ofthe temperature insensitive fiber is unaffected by change intemperature.
 2. A Mach-Zehnder interferometer according to claim 1wherein each of the two optical fibers is a temperature insensitivefiber.
 3. A Mach-Zehnder interferometer according to claim 2 wherein thetwo optical fibers are of different length.
 4. A Mach-Zehnderinterferometer according to claim 3 wherein each optical coupler hasarms of substantially equal length and with each arm connected to anindividual one of the two optical fibers.
 5. A Dense Wavelength DivisionMultiplex system comprising a Mach-Zehnder interferometer comprising twooptical couplers intercommunicated together by two optical fibers atlest one of which is temperature insensitive fiber in which temperatureinduced changes in the geometrical length and refractive index of thetemperature insensitive fiber offset each other whereby the optical pathlength of the temperature insensitive fiber is unaffected by change intemperature.
 6. A Mach-Zehnder interferometer according to claim 1wherein the temperature insensitive fiber is shorter than the otherfiber.
 7. A Mach-Zehnder interferometer according to claim 1 wherein thetemperature insensitive fiber is longer than the other fiber.